In recent years, the scanning electron microscope in many cases utilizes a low acceleration energy within a few kilovolts to prevent the electron beam from generating an electrostatic charge on the semiconductor sample. In these types of low acceleration SEM (scanning electron microscopes), the characteristic energy width of the electron source for the acceleration energy becomes relatively large so chromatic aberrations prevent obtaining an electron beam with a sufficiently small spot.
To solve this problem, technology utilizing an energy filter to selectively allow only electrons in the desired energy range to pass was disclosed in patent documents 1 through 5.
Patent document 1 discloses technology for reducing the chromatic aberrations by installing a magnetic field type energy filter between the electron source and the objective lens. This technology combines a magnetic sector field for lateral beam input/output, with a non-dispersive point (achromatic state) used as a crossover, along with an iris (beam limiter) installed at the beam convergence point along the linear optical axis of the beam emitted from the electron source.
The technology disclosed in patent document 2 utilizes a double structure Wien filter (overlapping magnetic field and electrical field) to monochromatize the electron beam, disperse the energy of the electron beam and select energy electrons with a single slit plate (FIG. 5 and FIG. 6 of patent document 2). The technology shown in FIG. 10 of patent document 2 discloses a monochromator composed of four magnetic sector fields. In this technology, after monochromatizing the electron beam with a slit installed downstream of two prestage magnetic sector field filters, the electron beam was non-dispersively converged by two final stage magnetic sector field filters.
The technology disclosed in patent document 3 describes an energy filter (monochromator) utilizing four semispherical electric deflectors (spherical electrical fields) and having slits in the mirror symmetrical surfaces midway between those deflectors. The beam emitted from the electron gun is subjected to energy dispersion by two prestage spherical electrical sector fields. Then, the energy width is reduced by the slits, the beam is non-dispersed by passing through two final stage spherical electrical sector fields, and a crossover is formed.
The technology disclosed in patent document 4 is not a monochromator but utilizes a Ω-type magnetic field filter installed in the projector lens system of a transmittance type electron scanning microscope. This energy filter further contains a linear path for the electron beam when not analyzing energy, and is further designed not to change the excitation of the front-rear lens.
The technology disclosed in patent document 5 is also not a monochromator and instead utilizes an energy filter composed of three sectorial magnets. One magnetic field is a uniform magnetic field for lateral input/output beam emission and works in combination with other non-uniform magnetic fields for energy selection.    Patent document 1:
JP-A No. 195396/1999    Patent document 2:
JP-A No. 357809/2001    Patent document 3:
JP-A No. 233145/1992    Patent document 4:
JP-A No. 191384/1999    Patent document 5:
JP-A No. 37536/1995